Attempts to grow virulent T. pallidum in vitro will be made using human cell cultivated in monolayers. Cell free and spent tissue culture medium will also be used to cultivate T. pallidum in a variety of environmental conditions. The virulent Nichols strain and fresh human isolates of T. pallidum will be employed. Tests for retention of virulence cultivated in vitro will be tested by serological procedures (FTA-ABS). Antigenic structure of virulent and avirulent T. pallidum cultivated in vitro and in vivo will be compared.